When a vehicle is braked, the brake forces introduced into the wheels cause a torque about the center of gravity of the vehicle. This results in an increase of the front-wheel load and a decrease of the rear-wheel load. Therefore, higher brake forces can occur at the front wheels than at the rear wheels. This has to be taken into consideration when dimensioning brake system. In the most simple case, the pressure which develops in the brake system is conducted to differently sized large surfaces, resulting in a linear relation between the brake forces at the front axle and at the rear axle. This relation is chosen such that, when a locking of the wheels cannot be avoided, first the front wheels and then the rear wheels will lock. This measure enhances considerably the directional stability of the vehicle during braking.
The desired brake force on the rear axle will not be achieved for a purely linear relation. Hence, proposals have been made to insert valves in the rear-wheel brake circuit which limit or reduce the pressure starting from a predetermined pressure point.
Further, anti-lock brake systems are known. In these brake systems, the rotational behavior of the wheels is monitored by means of sensors. The sensor signals are analyzed and processed to form control signals for a valve assembly composed of electromagnetically operable valves, by which the braking pressure is adjusted such that the wheels will not lock.
Further, German DE-OS 19 61 039 proposes using the solenoid valves not only for anti-lock control, but also for the control of the brake force distribution. To this end, the brake force at the front wheels and at the rear wheels must be measured directly or indirectly. From the comparison between the measured distribution and an optimal distribution, control signals for the solenoid valves of the rear-wheel brakes are derived.
Depending on how the linear brake force distribution inherent in the brake system is designed, it may happen that, on failure of the electronic control, the vehicle will reach an unstable condition during a braking operation because of locking of the rear wheels although the front wheels have not attained the locked condition.